A general network system uses a dependent transmission medium such as power line communication, Zigbee and RF to solve a wiring problem in an installation space and improve mobility. In addition, the network system using the dependent transmission medium such as the power line communication, Zigbee and RF divides networks (for example, households) by using network identifiers (home codes, etc.).
FIG. 1 is a block diagram illustrating the configuration of the general network system using the dependent transmission medium. Referring to FIG. 1, a household H1 and a household H2 are physically connected to each other through the power line which is the dependent transmission medium.
The household H1 is a network system composed of a network server 1a and electric home appliances 2a and 2b. The household H2 is a network system composed of a network server 1b and an electric home appliance 2c. When it is presumed that 0×11 is set as a home code of the household H1 and 0×01 is set as a home code of the household H2, a method for connecting a new electric home appliance 2d to the network system of the household H2 will now be explained.
In general, when a network identifier such as the home code is set, any home code is not stored in the electric home appliance 2d. When the electric home appliance 2d receives power, namely, when the electric home appliance 2d is connected to the power line, it receives the home code from the network server 1b, and sets the home code, thereby connecting the network. That is, the electric home appliance 2d is registered in the network of the household H2 and controlled by the network server 1b. 
In the above network connection method, when the household H1 and the household H2 intend to connect new electric home appliances to the networks almost at the same time, if the electric home appliance 2d receives the home code 0x11 of the household H1 before receiving the home code 0x01 of the household H2, the electric home appliance 2d sets the home code 0x11 as its home code. The electric home appliance 2d is controlled by not the network server 1b but the network server 1a, which causes a serious security problem.
This problem is not confined to the power line communication. It occurs in every network system using the dependent transmission media.
In the network system using the dependent transmission medium, if the network server 1b is quite distant from the network device 2d, the user must input network connection inputs to the network server 1b and the network device 2d, respectively. It takes a time for the user to move between the network server 1b and the network device 2d. As a result, the security problem grows more serious. Furthermore, the user must repeatedly move between the network server 1b and the network device 2d to make the inputs and check the network connection.